This invention relates to the field of semiconductor fabrication, and more particularly to a method and apparatus for attaching a semiconductor wafer section such as a die to a wafer section support such as a lead frame.
During the assembly of a multi-component semiconductor device, a semiconductor wafer section such as a die is typically attached to a wafer section support such as a lead flame, a ceramic support, or a printed circuit board. For example, a die attach adhesive such as a silver-filled epoxy paste is interposed between a die and a lead frame, then the lead frame is placed on a hot plate or another heat source for 2.5 hours or longer to cure the die attach adhesive. After this curing step the assembly is further processed, which includes attaching bond wires between the die and the lead frame, then encapsulating the die and a portion of the lead frame in plastic.
Increasing production output, which decreases the cost per device, is a continuing goal of process engineers. Adhesives which can be cured during the normal course of device assembly, or xe2x80x9cin-line curablexe2x80x9d adhesives, have been proposed. U.S. Pat. No. 5,475,048 by Jamison et al., issued Dec. 12, 1995 which is incorporated herein by reference in its entirety, describes a conductor-filled thermoset resin which can be cured in less than two minutes at a temperature of 200xc2x0 C. or below. Other adhesives which have a rapid cure profile given adequate temperatures are available.
Equipment is also available which provides rapid curing of adhesives. For example, the Sync-Rite SC-3000 oven available from Phicom USA, Inc. of Cupertino, Calif. contains eight programmable stages and has a cycle time of more than 80 seconds for each lead frame. This is a considerable time improvement over prior curing methods which require up to 2.5 hours but adds to equipment costs, processing steps, and space requirements.
One problem that often occurs with rapid curing such as that desirable with in-line cure processes is voiding in the adhesive material which results from outgassing during the adhesive cure. Voids form in the adhesive material lying between the two surfaces being attached, for example between a die and a lead frame, which provides a poor attachment that can later fail during operation of the device.
Additionally, excessive stress can be introduced to the package during a rapid cure process due to the rapid attainment of elevated temperatures and subsequent return to ambient in combination with large differences in the coefficients of thermal expansion of the package components, which can result in an early failure of the package or the semiconductor die itself. The in-line oven cure solution described above can reduce problems such as voiding and stress but, again, requires additional space, adds processing steps, and increases equipment costs.
A method and structure for attaching a wafer section to a wafer section support which increases throughput would be desirable.
The present invention provides a new method and apparatus used during the formation of a semiconductor device which increases production throughput by reducing the time required by the die attach procedure. In accordance with one embodiment of the inventive method, a die attach material is preheated to a first temperature over a first timed period. This first temperature is generally maintained for a second timed period at least 80% of the first timed period. Next, the die attach material is heated to a second temperature greater than the first temperature for a third timed period at least half as long as the first timed period.
Using such a temperature profile, many resin-based die attach adhesives can be cured in a very short time, for example in about four seconds, with acceptable levels of voiding therein. In another inventive embodiment, the die attach adhesive is cured in-line at a wire bond step using a preheater.
An inventive apparatus for heating a workpiece such as a die-lead frame assembly having an uncured die attach adhesive interposed therebetween may comprise a plate having a nonplanar face with at least first and second surfaces and a heater for heating the plate to a generally uniform temperature. The apparatus further comprises a transporter which advances the workpiece over the face along a generally planar path which does not contact at least the first surface. A first distance between the path and the first surface is greater than a second distance between the path and the second surface.
Objects and advantages will become apparent to those skilled in the art from the following detailed description read in conjunction with the appended claims and the drawings attached hereto.